1. Field of the Invention
This invention relates generally to the field of well logging. More particularly, the invention relates to the transfer and retrieval of data to and from a downhole tool used to measure subsurface properties.
2. Background Art
Modern petroleum drilling and production operations demand a great quantity of information related to subsurface properties and conditions. Such information includes characteristics of the formations traversed by the well bore, in addition to data relating to the size and configuration of the actual well bore. The collection of information relating to these subsurface properties is commonly referred to as “well logging.” Well logging operations are performed by several methods.
In “wireline” well logging, measurements are taken in a well bore (with the drill string removed) by lowering a logging instrument or tool into the well bore on an armored wireline cable and taking measurements with the suspended tool. Data is transferred between the suspended tool and the surface via the wireline cable.
Although wireline techniques have been the primary means for performing well logging for many years, the current trend is to perform the downhole measurements during the actual drilling of the well bore. This technique is referred to as “Logging-While-Drilling” or “Measurement-While-Drilling” [These terms are interchangeable and are referred to herein as (LWD)]. One of the primary reasons for this trend is the limitations associated with wireline logging. By collecting data during the drilling process, without the necessity of removing the drilling assembly to insert a wireline tool, subsurface data can be collected sooner and more economically.
The aim of LWD operations is to make downhole measurements of petrophysical, geological, mechanical and other parameters during the drilling process. The measurements are made using instruments disposed in the Bottom-Hole Assembly (BHA) of the drilling string. A part of the measured data is typically transmitted to the earth surface using a conventional telemetry system. However, due to bandwidth limitations in typical telemetry systems, only limited amounts of data can be transmitted between the surface and the tool during the actual drilling operation. In order to preserve much of the data collected during the drilling operation, a great deal of the data is stored in the tool until the instrument is brought back to the surface. Although this process may not be ideal, given the relatively slow data rates achievable in communications between downhole instruments and surface equipment, storing the collected data may be the only option for the majority of data.
With conventional data retrieval techniques, the stored data is retrieved from the tool memory when the tool is brought to the surface. At the same time, new parameter configuration data is often programmed into the tool memory to change the tool's mode of operation on the next drilling run. With conventional LWD tools, this operation of retrieving the data (or “Dumping” the memory) can cause significant disruption of the drilling process. Delay or disruption occurs because the rig has to remain inactive while the information in the memory is downloaded into the surface processing equipment. This process is especially expensive in offshore operations, which results in substantial economic loss.
In conventional tools, the downhole memory is typically downloaded to surface data processing equipment through a “Read-Out-Port” (ROP) on the side of the tool. This ROP typically comprises a connector internal to the tool and a hole in the collar through which the connector can be attached to the data processing equipment. A cable is used to connect surface equipment to the tool through the ROP. The hole in the collar is typically sealed with a pressure-tight insert before the tool is lowered into the well. One drawback of this system is that the tool has to remain immobile during the time needed to download the memory and reconfigure the tool. Increased data volume increases typical download times long enough to significantly impact the rig operations. Another drawback is the cable, which is a weak link in the system in terms of reliability and poses a safety hazard (tripping) to personnel.
U.S. Pat. No. 6,343,649 describes a technique for communicating with a downhole tool by conveying a service tool into the tubular string for engagement with a downhole communication device. U.S. Pat. No. 5,130,705 describes a self-contained data recorder for monitoring and collecting fluid dynamics data in a well pipe. U.S. Pat. No. 4,806,153 describes a technique for storing information about soil conditions using a cableless unit that includes a memory storage device adapted to collect the information throughout the drilling operation. After completion of the drilling process, the memory storage device is connected to a data processing unit to extract the collected information. U.S. Pat. No. 4,736,204 proposes using electromagnetic signals as a means for transmitting the stored data to a receiver mounted to the exterior of a logging tool. U.S. Pat. No. 4,928,088 (assigned to the present assignee) describes a technique using an electromagnetic link through an aperture in the side of a logging tool to establish a communications link between internal and external electronic systems.
GB 2358206 describes an LWD system that incorporates a stand-alone data download device. In this system, the data download device electrically couples to the tool and downloads data stored in the memory of the tool to a memory within the data download device. After the information is exchanged, the data download device can be de-coupled from the tool and physically carried to a location near the surface computer where logging information, now contained in the memory of the data download device, can be read by the surface computer.
These techniques continue to impose a delay to the drilling process while the data is manipulated and transferred. Thus there remains a need for a way to transfer data to and from a downhole tool, particularly during a drilling operation, in an efficient and expedient manner.